Composition of a solder, and method of manufacturing a solder connection

ABSTRACT

The solder composition comprises particles of a thermodynamically metastable alloy. One of the elements of the alloy will form an intermetallic compound with a metal surface. The solder composition is particularly suitable for use in bumping of semiconductor devices.

The invention relates to a composition of a solder.

The invention also relates to a method of manufacturing of anelectrically conducting connection between a bonding area at a firstsubstrate and a bonding area at a second substrate with a soldercomposition, comprising the steps of:

-   -   providing the solder composition onto the bonding area at the        first substrate;    -   assembling the first and the second substrate, such that the        solder composition is sandwiched between the said bonding areas        at the first and the second substrate, and    -   providing the electrically conducting connection by heating of        the solder composition.

The invention further relates to a substrate with a bonding area onwhich a layer of a solder composition is present and to an assembly of afirst and a second substrate provided with bonding areas that aremutually connected with an electrically conducting solder connection.

Solder compositions are known per se for the connection of metallicsurfaces. The metallic surfaces may extend over a large area, but arealternatively limited to small areas at an otherwise electricallyinsulating surface. The latter situation particularly occurs in thecontext of electronic components and products. An application ofspecific relevance is constituted by that of integrated circuits and thelike. These integrated circuits are increasingly connected to carriersin a flip-chip orientation with a plurality of solder or metal bumps.Trends herein are the use of a larger number of bumps per integratedcircuit and the reduction of the pitch between neighbouring bumps, andin particular the combination of both.

Requirements for the connection of electronic components to carriers areboth that of mechanically stability and electrically conductivity.Solder compositions fulfill these requirements in an excellent way. Thishas a number of reasons: the solder is first of all, electricallyconducting; better than alternatives as anistropically conducting glueand the like; then, the solder can be applied in ball-shapedportions—that are also referred to as bumps—; moreover, the solder bumpscan be applied before assembly and will flow out only at elevatedtemperatures under a heat treatment. It is therein even possible totransport a substrate with bumps. Said elevated temperature is at thesame time not so much elevated that the carrier or the components willbreakdown or are damaged. And finally, the solder connection can absorbpressures as a consequence of differences in thermal expansion betweenthe component and the carrier.

The presently available solder compositions however also havedisadvantages. One of those disadvantages is the need for adhesionlayers if the contact comprises a certain metal. Such metals are alsoclassified as badly solderable contacts. The badly solderable characteroften consists therein that an oxide surface layer is formed at thecontact. This oxide surface layer is electrically insulating andprohibits a direct connection between the solder and the metal. As aconsequence, both adhesion problems and conduction problems will appear.In order to solve such badly solderable contact, the contact isgenerally provided with adhesion layers. Such adhesion layers need to beapplied separately, which involves additional process steps andadditional costs. Moreover, the adhesion layers restrict the use of suchbadly solderable metals, particularly in that component and carrier needto withstand certain minimum temperatures. The complexity of the problemis moreover increased in that the contact at a first substrate comprisesa different metal that the contact at a second substrate that faces thefirst substrate.

It is therefor a first object of the invention to provide a soldercomposition that may be applied to establish an electrical connectionbetween bonding areas at two opposed substrates, at least one of whichcontacts is badly solderable.

It is a second object of the invention to provide a method ofmanufacturing an electrically conducting connection of the kindmentioned in the opening paragraph, which connection may be establishedonto a badly solderable bonding area without a separate application of aadhesion layer.

The first object is achieved in that particles of a thermodynamicallymetastable alloy are dispersed in the solder composition, which alloycomprises an element that on application of the composition to a metalcontaining surface will form an intermetallic compound with the metal ofsaid surface.

The second object is achieved in that a first of said bonding areas atthe first and second substrate contains an alloying metal and that asolder composition is applied, in which particles of a thermodynamicallymetastable alloy are dispersed, which alloy comprises an element thatwill form an intermetallic compound with the alloying metal of the firstcontact.

The solder composition of the invention is in fact a chemically stable,two-phase mixture of compounds that allow the provision of a goodelectrical connection to contacts at opposed substrates. But althoughchemically stable per se, the particles in the composition arethermodynamically metastable. This means that if the composition isheated, the constituents of the particles have the tendency to take on adifferent physical chemical shape, e.g. to diffuse into the soldercomposition and possibly to react. The constitution of the metastableparticles can therein be chosen such as to remove any oxide surfacelayer at the surface of a badly solderable contact. The oxide surfacelayer is then not only removed, but replaced by an electricallyconducting adhesion layer of an intermetallic compound. Thisintermetallic compound comprises both an element of the contact and anelement originating from the thermodynamically metastable particles. Theadhesion layer provides then the adhesion between the contact and thesolder composition. An important feature of the invention is moreoverthat any surplus of particles in the solder composition will aftercooling down continue to exist as a second phase in the solderconnection without substantial negative impact on the mechanicalstability or the electrical conductivity of the solder connection.

It is a first advantage of the present invention that the soldercomposition adheres at a large variety of different surfaces. Theseinclude, metals like aluminium, copper, gold, nickel and alloystherewith; semiconductor materials such as an ohmic contact of dopedsilicon; conductive oxides and nitrides such as indium-tin-oxide,ruthenium oxide and titanium nitride. Particularly advantageous is theapplication hereof in circumstances that do not allow a standard solderbump in view of the oxidizing surface, and particularly aluminium,nickel (that otherwise is provided with a gold surface), silicon andindium-tin-oxide. Major applications hereof are respectively labels andother conventional ICs; immersion soldering for fine pitch processing,particularly with a solder composition with a Sn-matrix such as BiSn orPbSn; die attach; and chip-on-glass processes and display drivers inparticular.

It is a second advantage of the present invention that the contacts atopposing substrates may contain a different metal as their primarycomponent. In this case, the adhesion layer may be formed at the contactof a single substrate only. Suitable examples include aluminum andcopper; aluminium and gold; indium-tin-oxide and copper or aluminum,etc.

It is a further advantage that the solder composition can be lead-free.The application of lead-free solder is required for environmentalreasons.

The particles of a thermodynamically stable nature that are applicablecomprise as alloying element for the intermetallic compound particularlyand preferably one or more of the group of indium, tin, bismuth andzinc. These elements can form intermetallic compounds with aluminum inparticular, but also with other elements such as tungsten, titanium,vanadium and nickel. In fact, the alloying element is herein chosen fromone of the higher groups of the periodic table (V, VI, Vb), whereas theelements present in the contact are in the lower groups of the periodictable (III, Ma, Iva), so as to enable to establish a noble gasconfiguration which forming the alloy.

In a further embodiment, the particles further comprise an element thatis able to form an oxide preferentially above aluminium oxide or tinoxide. This element is particularly chosen from the group of titanium,chrome, aluminum and nickel. An alternative approach is a treatment witha suitable acid solution for the removal of the aluminum oxid layer. Itis observed that aluminium can act as this component, notwithstanding analuminium oxide surface, since the aluminium oxide of the surface turnsout to have another energy level than that of the oxide particles formedfrom the metastable element. The resulting situation is a dispersion ofthese oxide particles in the solder bump.

Suitable compositions of the metastable particles are for instanceSnAg₄Ti₄, and ZnAl₆Ag₆. Although these particle compositions are knownper se, the use of these compositions as particles in a standard soldercomposition to obtain solder droplets that can be applied for bumping ofsemiconductor devices is not known. Moreover, the present use is anapplication to semiconductor devices with its specific requirements. TheSn-based particles herein have melting temperatures of 200-238° C., theZn-based particles have melting temperatures of 380-426° C. It has foundthat it is not necessary to heat the solder composition to a temperatureabove the melting temperature of the particles, but only to atemperature above the melting point of the matrix material.

In a particularly preferred embodiment the reacting element is Sn andthe solder composition comprises Sn. The presence of tin in both theparticles and the solution provides an improved stability of thecomposition. The use of tin-containing solder is furthermore wellestablished in the field of semiconductor assembly. Suitable examples ofsuch solders are the SAC (tin-silver-copper) solder, and tin-basedsolders such as SnCu, SnBi, PbSn, SnIn, SnZn and ternary or quaternaryalloys of tin, indium, bismuth and zinc. The combination of suchtin-containing solder with an metallization or bump of gold is mostpreferred, in that then an eutectic AuSn interconnect may be formed. Theuse of a tin-containing solder is furthermore advantageous in that ithas a relatively low melting point. As a consequence, the soldercomposition of the invention can be applied for badly solderablecontacts on carrriers that can withstand only a reasonably lowtemperature. Such carriers are particularly organic and flexiblecarriers.

Generally, the dispersed particles are present in a weight concentrationof 0.1 to 90%, and preferably in a weight concentration of 0.5 to 60%.The concentration is preferably on the lower side hereof, in the orderof up to 10%, as it is sufficient to form a thin layer of theintermetallic compound. The exact concentration may be chosen up todesire also in view of viscosity of the composition, and particle size.It will be understood that the rheology of the composition is animportant factor in order to ascertain that the surface of the contactwill be covered by the intermetallic compound as much as desired. It isof course highly preferred, both for the chemical stability and for theelectrical conductivity, that the complete contact is covered with theintermetallic compound.

The average diameter of the dispersed particles is for instance in therange from 0.1 to 80 μm. Preferably the average diameter is in the rangefrom 0.3 to 20 μm. The minimum size of the dispersed particles is onlydetermined by the tendency to dissolve in the solder composition, assuch a dissolution may change the phase of the elements in theparticles, and therewith the reactivity. It will be understood that asmall size is preferred in order to obtain a thin layer of theintermetallic compound with uniform thickness. Moreover, a small sizeenhances the uniformity of the distribution of the particles in thesolder composition. Additionally, small particle sizes appear to berequired for applications in which the pitch between individual dots ofthe solder is small. The pitch is generally defined as the distance fromthe center of one solder bump to the center of a neighbouring solderbump. Small herein implies a distance of 15 to 40 μm, and it will beunderstood that the particles need to be an order of magnitude smallerthan that at least, and preferably even more than that.

The composition of the invention is specifically suitable for use assolder bumps in electronic applications, in which the bonding areas arelimited to bond pads on an otherwise substantially electricallyinsulating substrate. Such bond pads generally have a dimension of lessthan 100*100 micrometers, including also less than 10*10 microns,however, they may be larger as well. Alternatively, the soldercomposition of the invention is used for other applications. Aparticular application is the die attachment of components to a carrier.Surprisingly good results have been obtained for the attachment of diescomprising a semiconductor substrate. Other applications include theconnection of two larger plates, the provision of rings of solder. Theparticle size may be adapted in this context so as to act as a spacerthat determines the distance between individual surfaces of two opposingsubstrates.

In the method of the invention, it is highly preferred that one of thebonding areas—in particular bond pads to which is also referred ascontacts—comprises aluminum. This metal is not only used as interconnectmaterial in integrated circuits and other components such as passivenetworks. It can be applied on labels and other flexible substrates atlow temperature and has, through its natural oxide layer a goodprotection against humidity and the like. However, the use of aluminumhas often provided problems and costs in that separate adhesion layershad to be applied, generally at higher temperatures. In the invention,this is not problematic.

It is moreover suitable, that the first contact is present at the firstsubstrate and the contact at the second substrate comprises a thickenedtop layer, that will form an alloy with the solder composition. Althoughthe reverse is not excluded, good results have been obtained inexperiments in which the intermetallic compound will form at thesubstrate at which the solder composition is applied. This has theadvantage that a stable connection and protection of the first contactcan be formed directly after the provision of the solder composition.The connection to the second substrate can be carried out at a laterstage, and possibly at another location. In view of the high stabilityof the intermetallic compound, this will not be redissolved in a latersoldering step; however, the layer of the intermetallic compound may bethickened as a consequence of continued deposition.

The presence of a thickened top layer leads to the formation of an alloywith the solder composition. Examples of thickened top layers includeunder bump metallizations, stud bumps, galvanic bumps and the like.Suitable materials are known per se to the skilled person and includefor instance nickel, copper and gold.

The method of the invention is particularly suitable for the provisionof components, such as integrated circuits on flexible carriers.However, it is certainly not limited to this application. Other suitableapplications comprise chip-on-chip applications, in which bond pads onone chip are connected to bond pads on another chip. Often, although notnecessary, at least one of these bond pads comprises aluminum or analuminum alloy. A first combination of chips is that of a memory chipand a processor or other logic chip. An alternative combination is anintegrated circuit with a network of passive components as a carrier. Afurther combination is for instance an integrated circuit with on top ofthat a passive component or network. Particularly for this lattercomponent the solder of the invention appears to be highly suitable, asthis needs to be a small pitch solution. Also, since no separateadhesion layer is needed, the costs are lowered and the flexibility isincreased. In a present version the metallisation layer of such apassive network is made of aluminum. The use of the composition of theinvention allows the reduction of mask steps substantially. Furthermore,the composition of the invention might well be tuned so as to havelimited wettability to aluminum or aluminum oxide only. In this manner,the solder composition can be applied maskless, and continued adhesionwill be established through the formation of the intermetallic compound.

Any heating needed for the formation of the intermetallic compound canbe provided locally or over the complete substrate. Use of a reflow ovenis an option but is not needed.

As stated above, the solder composition may be applied on a substratethat is thereafter sold or transported to another location. Theintermediate product is thus a substrate with the solder composition ofthe invention in the form of droplets or the like. The substrate willgenerally be part of the component, in casu the integrated circuit. Thepattern of contacts may be a ball grid array as generally used. It ishowever certainly not excluded that use is made of a pattern of bondpads on active.

The solder may be applied to the surface of the component in the form ofbumps, i.e. by individually depositing droplets. An alternative, butsuitable method is the use of immersion solder bumping. Herein theparticularly Sn-containing layer is provided on a suitable under bumpmetallisation, for instance of nickel or copper, by immersion. In thiscase, the opposite substrate may have the badly solderable contacts. Theadvantage of immersion soldering is the reduced pitch that can beachieved herein.

The invention further relates to an assembly of a first and a secondsubstrate provided with contacts that are mutually connected with anelectrically conducting solder connection.

It is a further object of the invention to provide such an assembly thathas an adhesion layer that provides good contact.

This object is achieved in that an adhesion layer comprising anintermetallic compound is present at an interface of at least one of thecontacts and the solder connection, said compound comprises an elementpresent in the contact and an alloying element originating from thesolder composition.

These and other aspects of the composition and the method of theinvention will be further elucidated with reference to the figures, inwhich:

FIG. 1 shows a diagrammatic cross-sectional view of the prior artsubstrate;

FIG. 2 shows a diagrammatic cross-sectional view of a substrateaccording to the invention;

FIG. 3 shows a diagrammatic cross-sectional view of the assemblyaccording to the invention;

FIG. 4 shows a photograph of a first test result

FIG. 5 shows a photograph of a second test result

FIG. 6 shows a photograph of a test result of a comparative example

FIG. 7 shows a diagrammatic cross-sectional view of a second example ofthe assembly of the invention; and

FIG. 8 shows a diagrammatic cross-sectional view of a third example ofthe assembly of the invention.

The Figures are not drawn to scale and the same reference numbers indifferent Figures refer to like parts.

FIG. 1 shows a diagrammatical cross-sectional view of a prior artsubstrate. This substrate is an integrated circuit device. It comprisesa plurality of elements, in this case a MOS transistor 2 and apolysilicon track 3. The circuit has been provided in an active circuitarea 4 at a surface of a semiconductor body 1. In overlying relationshiprelative to the circuit devices 2,3 an interconnect structure 8 isprovided for interconnecting the circuit devices 2,3 to form thecircuit. In this embodiment, the interconnect structure 8 comprises afirst patterned metal layer 5, a second patterned metal layer 6 andinterconnection vias 7. The patterned metal layers 5,6 comprise in thisexample Al or an aluminium alloy, such as AlCu. On top of theinterconnect structure 8 a layer of a passivating material 9 has beendisposed. The passivating material is for instance silicon nitride orsilicon oxide. The passivation structure may comprise further layersthat inhibit radiation and are chemically stable against reactiveetchants, so as to provide a hurdle against unauthorized access to theintegrated circuit. The latter is primarily important for application ofthe integrated circuit in smartcards and the like. By means of aphotolithographical steep and etching a via contacthole 10 has beenformed extending from the second patterned metal layer 6 and passingthrough the layer of passivating material 9. A barrier layer 11, forinstance comprising TiW or Ti/Pt is provided on the layer of passivatingmaterial 9 and in the via contact hole 10 for instance by means of asputtering process. The barrier layer 11 is relatively thin compared tothe layer of passivating material 9 and has a thickness of about 200 to300 nm. On top of the barrier layer 11 a metal layer 12 has beendeposited for instance by means of a sputtering process. This metallayer 12 may for instance comprise Au and has a thickness of 100 to 200nm. Subsequently a Pb/Sn bump 13 has been grown on the barrier layer 11and the metal layer 12 by means of electroplating preceded by aphotolithographical step to define the bump dimension. Together, thebarrier layer 11, the metal layer 12 and the bump 13 form the bumpelectrode. The bump electrode lies substantially straight above theactive circuit area 4.

FIG. 2 shows in diagrammatical cross-sectional view a substrate of theinvention. In this substrate, the barrier layer 11 and the metal layer12 are absent. Instead, use is made of the solder composition of theinvention as the material for the bump 13. This material is printed as asolder paste, in which the passivation layer 9 acts as the solder resistmaterial. Use is made of eutectic Sn₄₃Bi₅₇-soldering alloy as the matrixmaterial, comprising 4.5% by weight of particles of Sn₉₂Ag4Ti4 and 0.5%by weight of particles of Sn₉₀Al₆Ag₄. The solder composition is appliedin the contact holes 10 to the bond pads of Al or an Al alloy, which aredefined in the second patterned metal layer 6. These bond pads areprovided with a native aluminium oxide layer. The solder composition ismade to attach to the bond pads by a heating step to above the meltingpoint of the matrix material. The heating step was carried out at 170°C. in this example, as the melting point of eutectic tin-bismuth is 139°C. The resulting structure is applied to a carrier. The bond pads had asize of about 50×50 microns, which is however not critical and open toreduction.

FIG. 3 shows in diagrammatical cross-sectional view the assembly 100 ofthe invention. The integrated circuit 30 with the said solder bumps 13is herein applied to a carrier 20 comprising aluminium tracks 23 at afirst side 21 of the carrier 20. Subsequently, a bonding step is carriedout with ultrasonic bonding at 35 kHz at 5 W output, at a temperature of250° C. and during 10 seconds. The resulting connection comprisesadhesion layers 16, 26 of an intermetallic compound of SnAl.Additionally, particles 17 of SnTi are formed and present within thesolder bump 13 that bind the oxygen which is made free from theoriginally present aluminium oxide surface layer. Particles 18 of SnAgTimay be present in addition to the particles 17 of SnTi with incorporatedoxygen. These are the particles that did not or only partially reactduring the formation of the adhesion layer 16, 26.

FIG. 4 shows a photograph of a comparative example. In this case, usewas made of a SnBi alloy without any added particles. Cracks are visiblewhich show that the connection is not reliable. The black phase is Tinand the white phase is Bismuth.

FIGS. 5 and 6 show photographs of a cross-section of connections made infirst experiments. The difference between the photographs is in themorphology of the particles. FIG. 5 shows the result for needle-shapedparticles and FIG. 6 shows the result for particles with an irregularshape. The surfaces contain Aluminum. Two phases can be distinguished: aphase of the matrix material of SnBi, and a phase of the SnTi particlesformed, which particles incorporate the oxygen. The SnTi particles donot form an obstacle for the thermal or electrical conductivity of theconnection, as they are metallic particles. The adhesion layer of theintermetallic AlSn cannot be distinguished at the photographs.

The particles in this experiment had a diameter in the order of 10-20microns. This size will be reduced in further experiments to less than10 microns, more preferably less than 5 microns and most preferably inthe order of 1-3 microns. This reduction is done in view of theminiaturization in the IC industry. A well ohmic contact is obtained andthe conductivity is good.

FIG. 7 shows in diagrammatical cross-sectional view a second embodimentof the assembly 100 according to the invention. The solder composition13 of the invention is applied on a nickel or copper metallisation 12 inthis example. In this case use is made of the technique of immersionsoldering bumping as known per se. The nickel metallisation 12 ispreferably applied in an electroless process. This is a masklessprocess, and it can be applied with a very small pitch of less than 40microns and potentially in the order of 10 microns. The substrate 30 isthen applied to a carrier 20 comprising aluminium bond pads 24. Thiscarrier 20 is an integrated circuit in particular, having asemiconductor substrate 22, an interconnect structure 28 and apassivation layer 29. In view of the small height of the solder bumpapplied with immersion soldering, the bond pad 24 at the first side 21of the carrier 20 is provided on top of the passivation layer 29 bypreference, for instance in a bond pads on active process as known perse in the field. In such bond pads on active process, the bond pads 24are rerouted to an interconnect layer 25 below the passivation layer 29through vertical interconnects or the like. This embodiment has theadvantage over conventional immersion soldering bumping, that noadditional metallisation of Au need to be applied at the carrier 20.This again has the advantage that the resolution can be enhanced;contrarily to a metallisation, an aluminium bond pads on active layer 24can be suitable applied in a wafer fab and as part of standardprocessing. The immersion solder bumping technique has the advantagethat the soldering can be carried out without substantial pressure. Itis therefore very suitable for combination with bond pads on active,that cannot generally withstand high bonding pressures. Although notshown here, any space between the carrier 20 and the substrate 30 issuitably filled with an underfill. The bond pads 6 and solder bumps 13that are not connected to corresponding bond pads 24 at the carrier canbe used for connection to an external device or to other devices. Theconnection to an external device is suitably made with wirebonding orflip-chip, as known in the art. The substrate 30 may contain anintegrated circuit comprising active components such as transistors, butcould alternatively be a passive chip comprising capacitors, resistors,inductors and optionally diodes.

FIG. 8 shows in a diagrammatical cross-sectional view a furtherapplication of the solder composition of the invention. Herein, thesolder composition 13 is used for soldering of a backside 31 of asemiconductor device 30 to a heat sink 200 of the carrier 20.Particularly, the semiconductor device 30 comprises a silicon substrate1 as a support wafer. Additionally, it may contain buried layer ofanother material, such as an oxide, a nitride or even a silicide and/orlayers of another material, including SiGe, SiC and III-V materials. Thelatter materials may be grown epitaxially. Surprisingly, it was foundthat the solder composition 13 of the invention has a good adhesion tosilicon. In a first experiment, use was made of a solder composition 13on the basis of eutectic tin-bismuth solder. Similar results may beachieved with solders with a higher melting point, such astin-silver-copper, tin-silver and tin-copper alloys. As shown in thedrawing, the device 30 is provided with an interconnect structure 8 in atop layer of which bond pads 6 are defined. Exposed through apertures inthe passivation layer 9, that is present at a top side 32 of the device30, these bond pads 6 are provided with a metallisation 12, generallyreferred to as underbump metallisation (UBM) and with an electricallyconducting connection, in this case wirebonds 33. The wirebonds 33 areconnected to contact pads 201, 202 that are part of the carrier 20. Inthis case, the carrier 20 is a leadframe of the HVQFN-type. The assemblyis protected by an encapsulation 40 of any conventional material, suchas epoxy.

The invention claimed is:
 1. A structure comprising: a substrate with a bonding area comprising a metal that oxidizes on contact with air; a layer of a solder composition disposed over the bonding area, wherein particles of a thermodynamically metastable alloy are dispersed in the solder composition; and an adhesion layer comprising an intermetallic compound, the intermetallic compound comprising the metal that oxidizes on contact with air and an element of the thermodynamically metastable alloy, wherein the adhesion layer is disposed between the metal that oxidizes on contact with air and the layer of solder composition.
 2. The structure as claimed in claim 1, wherein the element is Sn and the solder composition comprises Sn.
 3. The structure as claimed in claim 1, wherein the dispersed particles are present in a weight concentration of 50% to 60%.
 4. The structure as claimed in claim 1, wherein the dispersed particles are present in a weight concentration of 10% to 90%.
 5. The structure as claimed in claim 1, wherein the dispersed particles have an average diameter between 0.5 and 80 mm.
 6. The structure as claimed in claim 1, wherein the dispersed particles have an average diameter between 1 and 20 mm.
 7. The structure as claimed in claim 1, wherein the element is chosen from the group consisting of Sn, Zn, In, Al and Bi. 